Lessons from a tiny, extremely destructive pest

May 13, 2014

The emerald ash borer is a little shiny green beetle that loves to feast on ash trees. The adult beetles only nibble on the leaves. It's the larvae you've got to watch out for. They munch on the inner bark of the ash tree, and mess with the tree's ability to transport water and nutrients.

Look at this cute little face! How bad could it be? BAD. Really bad.
Look at this cute little face! How bad could it be? BAD. Really bad.
Credit USDA Forest Service

The pest has killed tens of millions of ash trees in Michigan alone and tens of millions more in the states and provinces around our region.

Now researchers know a little bit more about how the emerald ash borer ate its way through the state.

The beetle is believed to have gotten into Michigan by catching a ride in wooden crating or pallets in cargo imported into the U.S.

Reading the tree rings

Deb McCullough is a professor of forest entomology at Michigan State University. She and her team took cores from more than 1,000 trees in southeast Michigan, and examined the tree rings.

"...one of the cool things that we found is that the emerald ash borer was killing trees in the Canton, and the area more or less along the I-275 corridor, by about 1997 and 1998."

She says they were able to take a marker year, say the 1998 drought, and see how many years the tree lived beyond that marker year before it was killed by the emerald ash borer.

“So, one of the cool things that we found is that the emerald ash borer was killing trees in the Canton, and the area more or less along the I-275 corridor, by about 1997 and 1998. And because those trees had already been killed by that point, we know ash borer must’ve been there for at least a few years before that, because it takes anywhere from four to five to six years before a population gets high enough to kill trees,” she says.

The new study is published in the journal Diversity and Distributions.

Michigan was the epicenter for the ash borer invasion. Since then, the pest has infested trees in 22 states and two Canadian provinces.

(You can learn everything you never knew you wanted to know about ash borers here.)

“I don’t think anybody believes you can stop emerald ash borer any more. There’s too many populations, the densities are so high in a lot of areas. But we’ve made some progress, and in terms of protecting landscape trees, we’ve made a lot of progress," she says.

She says there are new insecticides that can be injected into the base of the trunk that are much more effective than the products that were available when we were initially battling the ash borer in Michigan.

But what about this terribly cold winter? Didn't that affect the buggers?

“Not nearly as much as people would like to believe. If you think about ash borer larvae, they overwinter as immature larvae underneath the bark. The air temperature that you and I experience is not what larvae under the bark are experiencing. The bark and the wood provide some insulation. You have to get the entire tree really, really cold before it’s going to cause the larvae to die," she explains.

So how do we stop more pests from getting into the U.S.?

McCullough estimates around 1% of the cargo - at best - coming into the U.S. is inspected for pests.

“Our imports are increasing at roughly an exponential rate and that doesn’t show any sign of tapering off. The problem is certainly well-recognized, though. And especially when it comes to wood crating and wood pallet material, the federal government in the U.S. is aware of it. Some new regulations were implemented a few years ago, probably four or five years ago, that regulate how wood crating and wood pallets, that kind of material, has to be treated before it can be used in international trade."

She says those regulations are about 50% effective, though, so there's a lot more work to do to keep new pests out of our trees.